Re: [boost] [review] hash functions

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-03-13 10:57:08

Dave Harris wrote:

> My impression is that the proposed implementation of
>    hash_combine: seed ^= hash_value(v) + (seed << 6) + (seed >> 2);
>
> was designed to combine 8-bit character values; do we have any
> evidence it also performs well to combine 32-bit values?

(seed << 6) + (seed >> 2) does "neatly" expand an 8 bit value into a 14 bit 
one, but the seed is not necessarily an 8 bit value.

Think of the above as seed ^= hv + seed * 64.25.

But no, we don't have hard data on how it performs on 32 bit values.